Publication costs assisted by Universitat Bayreuth New developments of the theory of micellar kinetics are reported together with new experimental results obtained in the course of chemical relaxations (T-jump, p-jump, and shock tube) studies of micellar solutions of ionic surfactants. These results as well as those obtained in previous studies are quantitatively interpreted in terms of this theory. Several pieces of information thus far not available on micellar solutions have been obtained: (1) the rate constants k+ and k~for the association/dissociation reactions of one amphiphilic ion to/from micelles. The association reaction is very close to being diffusion controlled. The variation of the dissociation rate constant with the alkyl chain length is in quantitative agreement with what is expected from theory; (2) the width of the distribution curve of stable micelles is found to increase with chain length but the micelle polydispersity is small and decreases for increasing chain length; (3) an approximate value of the average number of amphiphilic ions in the micellar species at the minimum of the distribution curve; (4) the enthalpy and entropy changes associated with the incorporation of one amphiphilic ion into the most stable micelle and into the aggregate at the minimum of the distribution curve. The enthalpy changes for these two processes are of opposite signs. This explains why the overall heat of micellization is very small.

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The slow aggregation process of a concentrated silica dispersion (Bindzil 40/220) in the presence of alkali chlorides (LiCl, NaCl, KCl, RbCl, and CsCl) was investigated by means of mobility measurements. At intervals during the aggregation, particles and aggregates were transferred from the liquid phase to the gas phase via electrospray (ES) and subsequently size selected and counted using a scanning mobility particle sizer (SMPS). This method enables the acquisition of particle and aggregate size distributions with a time resolution of minutes. To our knowledge, this is the first time that the method has been applied to study the process of colloidal aggregation. The obtained results indicate that, independent of the type of counterion, a sufficient dilution of the formed gel will cause the particles to redisperse. Hence, the silica particles are, at least initially, reversibly aggregated. The reversibility of the aggregation indicates additional non-DLVO repulsive steric interactions that are likely due to the presence of a gel layer at the surface. The size of the disintegrating aggregates was monitored as a function of the time after dilution. It was found that the most stable aggregates were formed by the ions that adsorb most strongly on the particle surface. This attractive effect was ascribed to an ion-ion correlation interaction.